The present invention relates generally to molecular electronics, and, more particularly, to vapor-sensitive, molecular photodiodes based on thin films of certain platinum complexes.
The inventors have recently published reports that have enucleated and explained the unusual xe2x80x9cvapochromicxe2x80x9d changes in absorption and emission spectra that result when certain stacked platinum complexes are exposed to organic vapors; see, e.g., C. L. Exstrom et al, Chemical Materials, Vol. 7, pp. 15-17 (1995) and C. A. Daws et al, Chemical Materials, Vol. 9, pp. 363-368 (1997).
A typical experiment involves a solution, crystal or solid film of material, such as tetrakis(p-decylphenylisocyano)platinum tetracyanoplatinate (I) (see FIG. 1, which depicts the chemical formula of the compound, where the dashed vertical line indicates the c-axis) that forms stacks of alternating cations and anions with strong interplatinum interactions. These salts exhibit an intense absorption band in the visible region. Exposing the stacks to small molecule vapors, such as acetone or chloroform, leads to sorption of the vapor molecules in the free volume between the stacks, and produces shifts in the absorption and emission spectra. These xe2x80x9cvapochromicxe2x80x9d or xe2x80x9cvapoluminescentxe2x80x9d changes are usually reversible so that the original spectrum is regained quickly after the vapor is removed. Such an effect has potential application for sensor technology.
The inventors developed a new type of sensor technology, called the xe2x80x9cvapochromic LEDxe2x80x9d; see, Y. Kunugi et al, Journal of the American Chemical Society, Vol. 120, pp. 589-590 (January 1998) and application Ser. No. 09/225,758, listed above.
A sandwich LED was prepared using compound 1 that gave electroluminescence from this platinum compound; see, e.g., R. H. Friend et al in Physical Properties of Polymers Handbook, J. E. Mark, Ed., AIP Press (1996); Y. Yang, MRS Bulletin, pp. 31-38 (June 1997); T. Tsutsui, MRS Bulletin, pp. 39-45 (June 1997); W. K Salaneck et al, MRS Bulletin pp. 46-51 (June 1997); and C. Hosokawa et al, Synth. Met., Vol. 91, pp. 3-7 (December 1997). Exposure of the device to an organic vapor sharply changed the wavelength of electroluminescence, thereby providing a new method for remote vapochromic sensing which does not require a light source.
On the other hand, photodiodes, which do require a light source, have found extensive use in the electronics industry. Organic and polymer photodiodes have been built using materials such as poly(3-alkylthiophene)s, oligothiophenes, and C60. These photodiodes give photocurrents corresponding to the absorption of light by the molecular materials. They are of interest because they can give wavelength selectivity and quantum efficiencies of more than 10% electron/photon under modest reverse bias; see, e.g., H. Yonehara et al, Applied Physics Letters, Vol. 61, pp. 575-576 (August 1992); G. Yu et al, Applied Physics Letters, Vol. 64, pp. 422-3424 (June 1994); and Y. Kunugi et al, Chemical Materials, Vol. 9, pp. 1061-1062 (May 1997).
However, to the best of the present inventors"" knowledge, a photodiode which can detect the arrival of organic vapors has not been described. Such a device would be of interest in detecting the presence of organic vapors by a change in photocurrent.
In accordance with the present invention, a molecular photodiode is provided. The molecular photodiode employs an organic complex that acts as both a sensor to certain organic molecules, or analyte vapors, and as an active light emitter. The molecular photodiode of the present invention comprises:
(a) a first electrode;
(b) a first molecular layer formed on the first electrode, capable of at least transporting charge;
(c) a sensing/emitting layer formed on the first electrode, the sensing/emitting layer comprising a material that changes color upon exposure to the analyte vapors and that forms a rectifying junction with the first molecular layer; and
(d) a second electrode formed on the sensing/emitting layer, wherein at least the first electrode comprises an optically transparent material. The device is preferably formed on a transparent dielectric substrate, on which the first electrode is formed.
Also in accordance with the present invention, a method is provided for detecting analyte vapors. The method comprises:
(a) providing the above-described vapochromic photodiode;
(b) applying a voltage to the two electrodes in the range of 0 to about 50 V;
(c) shining light on the device in the wavelength range of about 300 to 1,000 nm;
(d) introducing the analyte vapors to the sensing layer; and
(e) measuring the photocurrent prior to and subsequent to exposure of the device to the analyte vapors to obtain a change in the photocurrent.
Further in accordance with the present invention, methods are provided for forming the vapochromic photodiode.
Other objects, features, and advantages of the present invention will become apparent upon consideration of the following detailed description and accompanying drawings, in which like reference designations represent like features throughout the FIGURES. The drawings referred to in this description should be understood as not being drawn to scale except if specifically noted.